Ah! My paste lost <id>, try this (next version will properly error)
CAGradientLayer gradientLayer = CAGradientLayer.layer();
gradientLayer.frame = butt.layer.bounds;
gradientLayer.colors = new NSArray WithObjects(bridge<id>(UIColor.blackColor.CGColor), bridge<id>(UIColor.greenColor.CGColor), null);
gradientLayer.locations = NSArray.arrayWithObjects(NSNumber.numberWithFloat(0), NSNumber.numberWithFloat(1), null);
gradientLayer.cornerRadius = butt.layer.cornerRadius;
butt.layer.addSublayer(gradientLayer);